1. Field of the Invention
The present invention pertains to optical telecommunication networks and in particular concerns WDM (Wavelength Division Multiplexing) networks. Furthermore in particular, the present invention is concerned with the virtual concatenation of the optical channels in such WDM networks.
2. Description of the Prior Art
As it is known for instance from the ITU-T Recommendation G. 707, the contiguous concatenation of a number X of Administrative Units 4 (AU-4-Xc) consists in concatenating together a number of AU-4s so as to transport together payloads (information part proper of an SDH frame) that require a greater capacity than the one provided by a single container having capacity 4 (VC4). In this way, the available capacity in the multicontainer 4 is X times the capacity of the single container-4 (for instance, 599040 Mbit/s for X=4 and 2396160 Kbit/s for X=16). In the concatenation of contiguous AU-4s, it is used a concatenation indication, contained in the pointer of the various AU-4s (except the first one), to indicate that the payload of the multicontainer-4 (carried in a single VC-4-Xc) should be kept together. The concatenation indication indicates that the pointer processors shall perform the same operations as performed on the first AU-4 of the AU-4-Xc. Therefore, the assumption made is that the various pointers are equal in such a way that, during the transmission in a network, shifts between one VC-4 and the other are not created, so as to be able to easily read the received data in the correct sequence. The network is required to transport information payload) without changing the value of the various pointers with respect to the first pointer (i.e. there is no reciprocal shifting).
Therefore, the principal objective of the concatenation is to transport, in an advantageous manner, the payload signals having bit rates different from the standard ones for synchronous hierarchy signals.
The so-called Inverse Multiplexing, typical of the ATM (Asynchronous Transfer Mode) transmissions, wherein pipes and high bit rates are transported by different independent signals E1, can be considered as an alternative technique. This technique can be regarded as an adaptation of the client layer to the transport means.
The objective of the concatenation is to provide those transport functionalities avoiding the need to adopt expensive equipment to carry out the adaptation. That is to say, it is the transport apparatus that performs the adaptation function that is not made at client level.
The virtual concatenation (ITU-T Recommendation G. 707) has been developed to provide concatenation functionalities in networks where the contiguous concatenation is not supported, thus avoiding the replacement of all the network elements. In the virtual concatenation, the pointer values shall no longer be strictly connected and, further, it does not exhibit the strictness of the contiguous concatenation that could be realized only with pre-established numbers X, for instance X=4, X=16 or X=64.
The virtual concatenation, therefore, does not affect the complexity of the network elements inside the network, but it complicates only the processing (with adaption operations) at the “peripheral” nodes of the transmission network. This results in an undoubted economic advantage since it may be adopted in all the existing networks without replacing the major parts of the nodes (all the nodes inside the network), merely replacing, or at least adapting, the network input/output peripheral nodes.
Another advantage of the virtual concatenation is that it can be adopted also with any Virtual Container (not only with the VC-4s) of the SONET/SDH hierarchies.
A flag is provided in the POHs (path Overheads) of the various virtually concatenated Virtual Containers (VCs). Such a flag is substantially a counter advancing at every step, i.e. at every POH. The network input mapping is like in the contiguous concatenation but, afterwards, the pointers are free to move relative to one another. Each VC has its own POH. A common counter provides a computation basis where the values are placed on all the POHs (in an identical way, namely the same value on all the POHs). Downstream of the transport network, the demapping occurs: the first VC of the concatenation reaches the value of the other VC counters and the shift accumulated among the various VCs in the transport network is determined. This shift (or staggering) is compensated for through the use of buffers. In this way, the client signal is provided perfectly equalized at the output.
The use of buffers can be considered the sole drawback of the virtual concatenation since it is added to the cost of the terminal nodes of the transport network: obviously this small complexity is negligible as compared with the cost for replacing/updating the nodes, should the concatenation not be used or the contiguous concatenation be utilized.
All the above relates to the SONET or SDH transport network, but the use of similar techniques in the art of WDM optical transport networks is not known. At present the sole possible way known to transport higher bit rates as compared with those carried by single wavelengths, is to split the information into several channels at client level. However, this technique is applicable only in end-to-end services and cannot be applied in the core of the transport network.